For example, Japanese Patent No. 1180690 (Japanese Patent Application Publication No. 52-072436) discloses a communication system (a time division multiple access transmission system). In this system, a communication control unit (a primary station) and communication terminal units (secondary stations) are connected to a communication line and communication is performed between the communication control unit and each communication terminal unit. The communication control unit is configured to regularly monitor each state of the communication terminal units and, if a communication terminal unit's state is changed (e.g., a switch included in a communication terminal unit is turned on or off), supply the other communication terminal units with a signal for performing a process corresponding to the state change. In the communication system, the communication is always performed via the communication control unit (A1) as shown in FIG. 1 and there is no chance that the communication terminal units (B1) directly communicate with each other.
Thus, the communication terminal units (B1) communicate with each other always via the communication control unit A1, and accordingly the communication system comes to have a low communication speed and is inept to transmission of information including comparatively a lot of data volume, such as a measurement value of electric energy. The whole system may also stop by failure of the communication control unit (A1) or the like, which causes reliability degradation.
Japanese Patent No. 3498646 (Japanese Patent Application Publication No. 2001-095232A) discloses a communication system. As shown in FIG. 2, this system is configured so that communication units (B2) connected to a communication line (W) directly communicate with each other by peer-to-peer (hereinafter also referred to “P2P”). In this configuration, the communication speed is improved, and accordingly it is possible to transmit information including comparatively a lot of data volume. In FIG. 2, A2 is a power feeding device that supplies electric power with each of the communication units (B2) via the communication line (W).
In these systems, the former is in widespread use compared with the latter, while the latter runs ahead of the former in point of a communication speed and reliability. Therefore, Japanese Patent Application Publication No. 8-274724 discloses a communication system including both the systems as shown in FIG. 3. In FIG. 3, each communication terminal unit (B1) generates control information in response to first monitoring information (e.g., switch on or off) to transmit the control information to one or other communication terminal units (B1) via the communication control unit (A1), thereby controlling the one or other communication terminal units' (B1) loads. Each communication unit (B2) receives second monitoring information (e.g., a measurement value of electric power consumption) and then directly transmits the second monitoring information to other communication units (B2) by P2P. Thus, in the communication system, the communication terminal units (B1) and the communication units (B2) operate independently, and the communication terminal units (B1) are used for load control, while the communication units (B2) are used for measurement or the like.
In the communication system of FIG. 3, when the second monitoring information, for example, a measurement value of electric power consumption of a load excessively increases, it is desirable that the electric power consumption of the load is reduced for power saving. However, in the communication system, the communication terminal units (B1) and the communication units (B2) are connected to the same communication line (W) but cannot communicate with each other, and accordingly cannot control each load of the communication terminal units (B1) based on the second monitoring information.